The 80% assumption is based on historical gender ratio of CS degree earned. Quick Googling shows historical percent at roughly 18% CS degree earned by women in 2014.

http://newsroom.ucla.edu/stories/cracking-the-code:-why-aren...

https://www.computerscience.org/resources/women-in-computer-...

https://www.usnews.com/news/data-mine/articles/2016-10-20/st...